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The espresso effect: caffeine and circadian rhythm

Your daily rhythms are influenced by “zeitgebers” such as light and exercise. But until now, we haven’t known the exact impact of late-day caffeine intake on melatonin and circadian rhythms.

Study under review: Effects of caffeine on the human circadian clock in vivo and in vitro

Introduction

Caffeine has been a component of the human diet for centuries[1], primarily through the consumption of coffee and tea. Culturally, coffee and tea are two of the main beverages that tie together many human interactions. Chemically, caffeine is well-known for being an adenosine receptor antagonist[2] that blocks the action of adenosine. Because adenosine mediates the perception of drowsiness, caffeine consumption results in alertness.

A circadian rhythm is essentially an organism's daily internal clock. It is not restricted to humans and exists in most living things, even bacteria[3]. For us, the circadian clock follows a roughly 24-hour cycle, responding primarily to light and darkness[4] in our environment, and acting to regulate countless bodily functions. One of the most important hormones in this context is melatonin, which is released by the pineal gland in the brain and is involved with sleepiness and sleep regulation.

Melatonin is secreted in response to darkness to help prepare the body for sleep. It is well-established that exposure to bright light suppresses[5] melatonin production. However, the ability of a stimulus to interfere with normal circadian function depends on the time of day. A pulse of blue light before bed will indeed suppress melatonin production and delay sleep, but that same pulse of light at noon will have no effect because melatonin production is low at noon anyway.

However, one recent population-based study[6] of adults estimated that 90% of individuals consume caffeine in the afternoon (12:00-6:00 pm) and 68.5% of people consume caffeine in the evening (6:00 pm - 12:00 am). Similarly, 37% of high schoolers who consume caffeine report[7] not using it until after 5:00 pm. Dose-response studies in humans[8] and animals[9] demonstrate that increasing doses of caffeine administered at or near bedtime are associated with significant sleep disturbance. Thus, the current study attempted to test whether nighttime caffeine consumption would delay the normal circadian rhythm of melatonin production, and evaluated the cellular mediators of any potential effect.

Research has backed the commonly seen experience of caffeine late in the day disrupting sleep and hence circadian rhythms. The specific mechanisms for this, as well as impact on the sleep hormone melatonin, are not fully understood though.

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